1. Field of Invention
The invention relates to a protective barrier against falls of stones on sloping ground, comprising at least one sheet of netting or other suitable material substantially following the ground traverse to the slope and held spread out upright by rigid posts and retained at least upstream by mooring cables disposed in line with the posts and connected to anchorages, the cables comprising shock-absorbing means adapted to supply a length of reserve cable in response to a tensile force exceeding a set value.
2. Description of Prior Art
There have been numerous descriptions and material embodiments of protective barriers against falling stones, comprising a grid or net stretched on posts across the anticipated path of the stones along sloping ground. The main difficulty of this kind of barrier is to maintain protection after the impact of large blocks. It is of course economically impracticable to dispose barriers, walls, or brackets upstream of the places to be protected and capable under all circumstances of stopping the largest blocks which may fall down the upstream slopes, without the need to overhaul the protective obstacles. A first step towards a solution is to use netting or a mesh network which distributes the energy of impact over regions adjacent the actual place of impact. However, particularly when the impact occurs near a post, the forces resulting from the impact are concentrated on the post, which is subject to forces pulling it over downstream. Heavy impacts will cause the post to bend or break near its base, which is set in the ground. One idea has been to use springs to attach a sheet of netting to posts, to reduce the abruptness of impacts. However, the reduction is limited owing to the small amount by which the springs can stretch. Also, springs do not absorb the energy of impact and there may be dangerous rebounds.
French patent application No. 2 414 586 describes a protective safety barrier, inter alia against falling stones, which comprises a mesh screen held stretched by cross-members, with respect to which the network is enabled to move by connecting means of limited strength extending between the screen and the places where it is anchored and retained. The connecting means are cables forming stays and extending through apertures at the top and bottom of the posts or cross-members and connected to upstream anchorages. The cables have tension-limiting devices adapted to allow portions of reserve cable to run between jaws which are adjustably tightened to define a resistance threshold.
The side cables of the mesh screen may comprise tension limiters of the same kind suitably adjusted.
During the impact of a large block, the tension limiters yield, the energy of impact being used up in friction by the cables in the tension limiters. The screen moves away from the posts by an amount equal to the length of yielded stay, and tends to close again in a pocket around the block, since the stay portions which have run in the post apertures enable the edges to come together to a certain extent.
This kind of barrier has been found effective but still has some drawbacks. If a large block directly strikes a post, the post will bend or break and the barrier will collapse downstream and thus become completely ineffective at the place where the post was struck. Furthermore, repeated impacts in a localized area cause the edges of the mesh screen to come together behind the accumulated blocks and reduce the effective height of the screen, with the risk of forming a reverse-gradient slope which can be crossed by stones subsequently falling down the slope.